This invention relates to polyvinyl chloride (PVC) pipe and, in particular, a PVC pipe comprising a processing aid and an external lubricant.
Oriented polyvinyl chloride (o-PVC) pipes offer significant improvements over non-oriented PVC pipes by maintaining burst and impact strength of the pipe while reducing its weight. o-PVC pipe can be prepared by extruding a non-oriented pipe at about half the desired final diameter, then stretching the pipe in the radial direction to about twice the original diameter. The process provides a pipe wall thickness that is reduced by about half, with concomitant alignment (orientation) of the PVC chains, resulting in an increase in the strength of the PVC material. Thus, an o-PVC pipe is roughly half the weight and requires half the raw material of a regular PVC pipe, while maintaining or improving strength over the non-oriented pipe.
Despite its huge improvement over non-oriented PVC, o-PVC has encountered obstacles due to the economics of the process. Typically, o-PVC is made in a semi-batch process, wherein the pipe is extruded, cut into sections, heated to 100° C., and pressurized to expand. The labor and equipment required in such a process greatly increases the cost of o-PVC pipe. Alternatively, a continuous process can be used, wherein the extruded pipe is cooled to 100° C. and stretched over a mandrel. Though less expensive, the continuous process can only run at one third the rate of the extrusion process, which is limited by breaking of the PVC melt during stretching before achieving the full strain necessary to double the pipe diameter.
Process aids are known in the art. For example, U.S. Pat. No. 4,692,295 discloses certain PVC process aids containing a homopolymer of a vinyl monomer or a copolymer of at least two vinyl monomers for achieving orientation in a thermoplastic resin. U.S. Pat. No. 6,391,976 discloses a PVC foam process aid containing a powdered copolymer of methyl methacrylate and a C3-C5 methacrylic ester.
Nevertheless, there is a need to develop PVC formulations that give improvements in melt strength and improved formulation processibility, which would be expected to provide an increased run rate in the continuous stretching of PVC pipe.